Turbofan engines are frequently employed in aviation. A common feature of modern turbofan engines is an arrangement for varying the angles of vanes throughout. Conventionally, there are at least two general methods and/or systems for controlling variable pitch vane stators. First, all variable stator vanes of a single stage are coupled to a unison/synchronization ring, for example by a lever, and the unison/synchronization ring may then be linked to a torque tube such that the number of actuators needed to manipulate all stators for a single compression stage is decreased or minimized. According to a second alternative, all variable stator vanes of a single stage are again coupled to a unison/synchronization ring by a lever, or another suitable coupling, and a cam plate driven by one or more actuators is attached to the unison/synchronization ring in order to effectuate rotation of the variable vane stators as a result of the actuated movement.
Both conventional methods and/or systems face challenges in achieving accurate positioning of the variable stator vanes. Generally, a turbofan engine subsystem arrangement for controlling variable stator vanes is constrained by manufacturing tolerances, variation in assemble, setup and/or initialization effects, thermal effects, synchronization of multiple actuators, unison/synchronization ring deflection, and scheduling effects. All of these constraints contribute to the cumulative accuracy tolerance of the sub-system controlling the position of the variable stator vanes. Accordingly, there exists a need in the field of engine control, specifically control of axial compressors found in turbofan engines, for the subject matter of the below disclosure that increases the relative accuracy with which variable vane stators may be positioned. While this disclosure uses a turbofan engine as an exemplary embodiment, the subject matter therein is not limited to use with axial compressors or turbofan engines, but instead may be applied more broadly to the field of engine control.
Under the ideal operating conditions, conventional axial flow compressors having variable pitch stator vanes function such that all the stages of variable stator vanes are operating at a desirable, possibly maximum, efficiency and each stage of stator vanes has a surge margin. When the turbofan engine, and therefore the rotor of each compressor, is operating at rotational speeds lower than the ideal rotational speed of the compressor rotor it is beneficial to vary the angles of the stator vanes to prevent surge or stall of the compressor. However, manipulation of the angles of the stator vanes according to conventional methods may exacerbate any encountered surge or stall.
A turbofan gas turbine engine 10 is shown in FIG. 1 and comprises a fan assembly 15 and a compressor assembly 22. The fan assembly 15 comprises a plurality of fan blades 24 secured to and extending radially from a fan rotor 25. The fan blades 24 and fan rotor 25 are enclosed by a fan casing 26. The compressor assembly 22 includes alternating rotor blades stages and stator vane stages (FIG. 3) disposed along an axis of the turbofan engine 10. In the illustrated example, there are five stages of variable pitch stator vanes. Each of the stages of variable stator vanes comprises a plurality of circumferentially arranged radially extending stator vanes 14. The stator vanes 14 are mounted for rotation about their longitudinal axis within a compressor casing 33. A plurality of control/synchronization rings 34, 36, 38, 40, and 42 are arranged substantially coaxial with the compressor assembly 22, and around the stator casing 33. The variable pitch stator vanes 14 at each stage are connected to the associated one of the plurality of control/synchronization rings 34, 36, 38, 40, and 42 by levers 44 which are shown more clearly in FIG. 2.
Referring now to FIGS. 1 and 2, each of the control/synchronization rings 34, 36, 38, 40, and 42 is connected to an axial control beam 64 by operating links 54, 56, 58, 60, and 62, respectively. The operating links 54, 56, 58, 60, and 62 extend substantially tangentially with respect to their associated control/synchronization rings 34, 36, 38, 40, and 42, and the axial control beam 64 is arranged substantially perpendicular to the control/synchronization rings 34, 36, 38, 40, and 42. One or more actuators, such as first and second hydraulic actuators 66, 68 are provided to move the axially extending member 64. The first and second hydraulic actuators 66, 68 are connected to the axial control beam 64 and spaced therealong.
One or more position detector(s) or other sensor(s) 78, 80 are located on one or more associated control rings 34, 42. The position detectors/sensors 78, 80 detect the pitch or angle setting of the vanes and are arranged to produce electrical signals that are transmitted to a controller 94 via cables 82, 84. Alternatively to, or in combination with, the one or more position detector(s)/sensor(s) 78, 80, one or more position detector(s)/sensor(s) 86, 88 may be located on the hydraulic actuators 66, 68. The position detectors/sensors 86, 88 detect the position of the pistons in the hydraulic actuators 66, 68, and are arranged to produce electrical signals that are transmitted to the controller 94 via cables 90, 92.
The controller 94 uses the feedback from the position detectors/sensors to determine the position of the pitch of the vanes 14 in each variable vane stator stage and to determine the position of the pistons in the hydraulic actuators 66, 68. The controller 94 may determine necessary adjustment to the stages of variable stator vanes in order to match the engine parameters with the desired characteristics/outputs of the engine, and the controller 94 controls the flow of hydraulic fluid to the hydraulic actuators 66, 68.
In operation the axial control beam 64 is moved in a plane substantially tangential to the control rings 34, 36, 38, 40, and 42 to rotate the control rings coaxially of the compressor such that the pitch of the variable stator vanes 14 in the variable vane stator stages is manipulated. The hydraulic actuators 66, 68 may be moved in unison in the same direction such that there is a proportional movement of the variable stator vanes 14 in each stage, or, alternatively, may be moved non-matching distances such that there is a non-proportional movement of the variable stator vanes 14 across different stages.
In FIG. 3 the compressor assembly 22 of a turbofan engine 10 is shown. Within the compressor casing 33 of the compressor assembly 22 are mounted plural stages of variable stator vanes 14 circumferentially about the central axis of the compressor assembly 22. A corresponding set of rotor blades 16 is mounted downstream of each set of stator vanes 14. Each stator vane 14 terminates at the casing 33 in a stem 18 rotatable in a bush bearing 20 on the outside of the casing wherein the end of the stem extends beyond the bush.
Located externally of the casing 33 and adjacent each set of stator vanes 14 are control/synchronization rings 34, 36, 38, 40 (control/synchronization ring 42 is not shown in FIG. 3) extending circumferentially round the compressor casing 33. For each variable stator vane 14 in a stage, the vane is connected to a corresponding control/synchronization ring 34, 36, 38, 40, 42 by a plurality of levers 44. One end of each lever 44 is clamped to the end of the variable stator vane 18 by a bolt so that there is no relative movement between the vane 14 and the lever 44. The other end of the lever 44 is connected to the control/synchronization rings 34, 36, 38, 40, 42 by a pin 28 rotatable in a bush bearing located within the respective ring.
The control/synchronization rings 34, 36, 38, 40, 42 are arranged so that each ring may be rotated about the central axis of the compressor assembly 22, i.e. in either direction of arrow 9. Consequently, rotation of each control/synchronization ring(s) 34, 36, 38, 40, 42 will, by means of the levers 44, cause rotation of each variable stator vane 14 about its own axis and thus enable the vanes 14 to assume required angles of incidence to incoming air.
A disadvantage of the geometry of this arrangement is that, as the control/synchronization rings 34, 36, 38, 40, 42 rotate, there is a tendency for each lever 44 to rotate about its longitudinal axis and, because of its stiffness, to loosen the retaining bolt. It is further necessary for the levers 44 to resist surge loads in the engine. Until now these problems have been solved by providing forged vane levers 44. However, these are costly, need extensive machining, and generate a weight penalty.
It is therefore desirable to provide a variable stator vane positioning system and device that provides improvements in accuracy, cost, replacement time and expense for stator vanes and the vane positioning system itself, and overall variable vane stator control.